A direct oxidation fuel cell (DOFC) is an electrochemical device that generates electricity from electrochemical oxidation of a liquid fuel. DOFC's do not require a preliminary fuel processing stage; hence, they offer considerable weight and space advantages over indirect fuel cells, i.e., cells requiring preliminary fuel processing. Liquid fuels of interest for use in DOFC's include methanol (CH3OH), formic acid, dimethyl ether (DME), etc., and their aqueous solutions. The oxidant may be substantially pure oxygen (O2) or a dilute stream of oxygen, such as that in air. Significant advantages of employing a DOFC in portable and mobile applications (e.g., notebook computers, mobile phones, PDA's, etc.) include easy storage/handling and high energy density of the liquid fuel.
One example of a DOFC system is a direct methanol fuel cell (DMFC). A DMFC generally employs a membrane-electrode assembly (hereinafter “MEA”) having an anode, a cathode, and a proton-conducting membrane electrolyte positioned therebetween. A typical example of a membrane electrolyte is one composed of a perfluorosulfonic acid-tetrafluorethylene copolymer, such as Nafion® (Nafion® is a registered trademark of E.I. Dupont de Nemours and Company). In a DMFC, a methanol/water solution is directly supplied to the anode as the fuel and air is supplied to the cathode as the oxidant. At the anode, the methanol (CH3OH) reacts with the water (H2O) in the presence of a catalyst, typically a Pt or Ru metal-based catalyst, to produce carbon dioxide (CO2), protons (H+ ions), and electrons (e−). The electrochemical reaction is shown as equation (1) below:CH3OH+H2O→CO2+6H++6e−  (1)
During operation of the DMFC, the protons migrate to the cathode through the proton-conducting membrane electrolyte, which is non-conductive to electrons. The electrons travel to the cathode through an external circuit for delivery of electrical power to a load device. At the cathode, the protons, electrons, and oxygen (O2) molecules, typically derived from air, are combined to form water. The electrochemical reaction is given in equation (2) below:3/2O2+6H++6e−→3H2O  (2)
Electrochemical reactions (1) and (2) form an overall cell reaction as shown in equation (3) below:CH3OH+3/2O2→CO2+2H2O  (3)
One drawback of a conventional DMFC is that the methanol partly permeates the membrane electrolyte from the anode to the cathode, such permeated methanol being termed “crossover methanol”. The crossover methanol reacts with oxygen at the cathode, causing a reduction in fuel utilization efficiency and cathode potential, with a corresponding reduction in power generation of the fuel cell. It is thus conventional for DMFC systems to use excessively dilute (3-6% by vol.) methanol solutions for the anode reaction in order to limit methanol crossover and its detrimental consequences. However, the problem with such a DMFC system is that it requires a significant amount of water to be carried in a portable system, thus diminishing the system energy density.
The ability to use highly concentrated fuel is desirable for portable power sources, particularly since DMFC technology is currently competing with advanced batteries, such as those based upon lithium-ion technology. However, even if the fuel cartridge with highly concentrated fuel (e.g., pure or “neat” methanol) carries little to no water, the anodic reaction, i.e., equation (1), still requires one water molecule for each methanol molecule for complete electro-oxidation. Simultaneously, water is produced at the cathode via reduction of oxygen, i.e., equation (2). Therefore, in order to take full advantage of a fuel cell employing highly concentrated fuel, it would be desirable to: (a) maintain a net water balance in the cell where the total water loss from the cell (mainly through the cathode) preferably does not exceed the net production of water (i.e., two water molecules per each methanol molecule consumed according to equation (3)), and (b) transport some of the produced water from the cathode to anode.
Two approaches have been developed to meet the above-mentioned goals in order to directly use concentrated fuel. A first approach is an active water condensing and pumping system to recover cathode water vapor and return it to the anode (U.S. Pat. No. 5,599,638). While this method achieves the goal of carrying concentrated (and even neat) methanol in the fuel cartridge, it suffers from a significant increase in system volume and parasitic power loss due to the need for a bulky condenser and its cooling/pumping accessories.
The second approach is a passive water return technique in which hydraulic pressure at the cathode is generated by including a highly hydrophobic microporous layer (MPL) in the cathode, and this pressure is utilized for driving water from the cathode to the anode through a thin membrane (Ren et al. and Pasaogullari & Wang, J. Electrochem. Soc., pp A399-A406, March 2004). While this passive approach is efficient and does not incur parasitic power loss, the amount of water returned, and hence the concentration of methanol fuel, depends strongly on the cell temperature and power density. Presently, direct use of neat methanol is demonstrated only at or below 40° C. and at low power density (less than 30 mW/cm2). Considerably less concentrated methanol fuel is utilized in high power density (e.g., 60 mW/cm2) systems at elevated temperatures, such as 60° C. In addition, the requirement for thin membranes in this method sacrifices fuel efficiency and operating cell voltage, thus resulting in lower total energy efficiency.
Thus, there is a prevailing need for a direct oxidation fuel cell system that maintains a balance of water in the fuel cell and returns a sufficient amount of water from the cathode to the anode under high-power and elevated temperature operating conditions. There is an additional need for a direct oxidation fuel cell that operates with highly concentrated fuel, including neat methanol, and minimizes the need for an external water supply or condensation of electrochemically produced water.
In this regard, it is disadvantageous to store very highly concentrated fuel (e.g., from about 17 M to about 25 M methanol, i.e., neat methanol) in a fuel container or cartridge supplying the DMFC because operation with highly concentrated fuel necessitates operation at low oxidant stoichiometric ratios, e.g., less than about 2, and the electrical performance, e.g., in terms of power, declines as the oxidant stoichiometric ratio becomes smaller. In addition, the performance of the DFMC is very sensitive to variations in the air flow rate, especially at such lower oxidant stoichiometric ratios. As a consequence of these apparently competing factors, it is difficult to simultaneously achieve high power output and very high concentration fuel use with present DOFC/DMFC systems. Further in addition, it is considered important from health and environmental standpoints that the amount of undesirable substances contained in the exhaust gases from the DOFC/DMFC system be minimized, i.e., the exhaust gas from the system should be as pure as possible.
In view of the foregoing, there exists a clear need for improved DOFC/DMFC systems and methodologies which eliminates, or at least mitigates, the effects of these competing factors tending to prevent development of systems operating with very highly concentrated fuel at high conversion efficiency. In addition, there exists a clear need for DOFC/DMFC systems and methodologies which minimize the amount of environmentally undesirable substances in the exhaust gas.